The present invention is directed to a fiber-optic repeater, and more particularly to a T-connection fiber-optic repeater for use in a multi-star fiber optic communication network in order to avoid repeated signal return from star to star.
Continuing improvements in the transmission quality of optical fibers, and in particular increased bandwidth and reduced attentuation rates, have made optical fiber communication networks an increasingly attractive alternative to networks which employ conductors as the transmission medium. In order to communicate optically, an electrical signal developed within a transmitting terminal device such as, for example, a telephone, computer, or numerically controlled machine tool, is delivered to an optical transmitter within the terminal device. The optical transmitter uses the electrical signal to modulate light from a source such as an LED or a laser. The modulated light is then transmitted via an optical fiber to an optical receiver within a receiving terminal device. The optical receiver includes an optical detector, such as a photodiode, which reconverts the modulated optical signal into an electrical signal. Thus the optical transmitters and optical receivers within the terminal devices, together with the optical fibers connecting them, effectively replace conductors which might otherwise have been used. Optical fibers are particularly useful when digital data in serial form are to be transmitted.
A transmissive fiber optic star is a passive coupling device used to interconnect a number of terminal devices in a network. The physical structure of such a star is illustrated schematically in FIG. 1, wherein four optical fibers have been fused at a tapered region 20 to provide star 22 having light input ports 24, 26, 28, and 30 and light exit ports 32, 34, 36, and 38. LIght entering star 22 through any of the input ports 24-30 is equally distributed to all of the exit ports 32-38. For example, if light having an intensity of one unit were introduced into input port 24, light having an intensity of one-quarter unit (neglecting minor loses) would be emitted through each of exit ports 32-38. Star 22 could be used to interconnect four terminal devices, each terminal device being separately connected via optical fibers to one of the input ports and one of the exit ports.
Stars are not limited to four pairs of ports, as in the example of FIG. 1. However the number of terminal devices that can be interconnected via a single star is generally under 80. This limitation is caused partially by difficulties in fabricating larger stars (i.e., stars with more than 80 pairs of ports). Another limiting consideration is that the optical power available at each light exit port is inversely proportional to the total number of exit ports. Thus the available sensitivity of the optical receivers effectively imposes a limitation upon the star itself.
FIG. 2 schematically illustrates a fiber-optic communication network employing a star 40. In this Figure, terminal device 42 in Local Area A is connected to a light input port and a light exit port of star 40 by fibers 44 and 46, respectfully. Similarly, terminal devices 48 and 50 in Local Area B are connected to star 40 by fibers 52, 54, 56, and 58. Each terminal device has an optical transmitter which receives electrical signals in serial, digital form and transforms them into corresponding optical signals and an optical receiver which receives optical signals and transforms them back into electrical signals, although only transmitter 60 and receiver 62 in terminal device 42 are illustrated in the drawings. FIG. 3A illustrates a simple example of an optical transmitter which might be employed as transmitter 60. Input terminals 64 receives a digital signal in serial form. This signal is amplified by driver amplifier 66 and then provided to LED 68, which flashes ON and OFF in synchronism with the electrical code provided by the terminal device to terminal 64. These flashes enter the end of optical fiber 44. FIG. 3B illustrates a simple example of circuitry suitable for use as optical receiver 62. Flashes of light exiting the end of fiber 46 impinge upon an optical/electrical transducer such as photodiode 70. The output is amplified by amplifier 72 and provided to waveshaping circuit 74, such as an comparator or Schmidt trigger. Circuit 74 imparts sharp leading and trailing edges to the signal, which is provided to the terminal device via output terminal 76.
It should be noted that various sophisticated digital communication techniques which have been developed in the electrical communication art have been adapted for use in optical networks. Referring again to FIG. 2, it will be apparent that communication chaos would result if terminal devices 42, 48, and 50 were permitted to transmit simultaneously. This problem arises regardless of whether the transmission medium consists of conductors or optical fibers. Various network control systems have been developed to allow only one terminal device at a time access to the network for purposes of transmitting. For example in a poling system, a central network manager sequentially emits codes which identify each terminal device. If a terminal device has a message to send, it waits until it receives its identification code. In the token passing system, the function of the central network manager is distributed to the individual terminal devices. The identification codes are known as "tokens," and a terminal device having access to the network "passes the token" to the next terminal device entitled to access after sending any messages it may have. In the collision detection system, each terminal device monitors the network and is permitted to transmit at any time the network is not already in use. This occasionally results in simultaneous transmissions, and these "collisions" are detected by the transmitting terminal devices. The transmitting terminal devices then abort their transmissions and try again after a random delay. Such network control systems can be implemented electronically within the terminal devices connected to a fiber-optic network.
FIG. 2 illustrates a primary problem which is encountered in single star fiber-optic networks. If the terminal devices are widely dispersed, a large amount of fiber is required to run a separate pair of fibers from the star to each terminal device. This increases cabling complexity and network costs. For example if Local Area A represents a suite of offices in one building and Local Area B represents a suite of offices in a building a block away, an appreciable amount of fiber would be required to interconnect as few as ten terminal devices in Local Area A and another ten terminal devices in Local Area B. It will be apparent that, although the schematic symbol for star 40 might suggest that only four pairs of ports are present, which could be used to interconnect only four terminal devices, no such limitation is intended. As was mentioned above the capacity of the star is frequently significantly greater, and in practice star 40 would typically be used to interconnect more than the three terminal devices illustrated in FIG. 2.
Turning next to FIG. 4, one might attempt to reduce the amount of fiber required to interconnect a plurality of terminal devices in different local areas, such as terminal devices 78 and 80, by using a pair of stars 82 and 84, a light exit port of one star being optically connected to a light input port of the other star, and vice versa. However if star 84 had N pairs of ports, only 1/N of the optical power provided by terminal device 78 would be delivered to star 84. If star 84 also had N pairs of ports, it will be apparent that the optical power provided to terminal device 80 would be only 1/N.sup.2 of the optical power originally delivered by terminal device 78. The signal attenutaion would be even greater if there were more than two stars in the sequence.
In order to avoid this problem of signal attenuation, one might seek to insert repeaters 86 and 88 into the optical fibers connecting stars. Each such repeater would have an optical receiver portion (which might be the circuitry in FIG. 3B) to receive incoming optical signals and regenerate the original electrical signal, and an optical transmitter portion (such as the circuitry illustrated in FIG. 3A) to convert the regenerated signal back into optical form. This solution to the attenuation problem, unfortunately, would create its own problem. The optical output from repeater 86, for example, would be received by star 84 and distributed to each of its output ports, one of which is connected to repeater 88. Repeater 88 would launch the signal toward star 82, which would thereupon return it back to repeater 86. The result would be endless signal "reflection" between stars 82 and 84.
In order to avoid this reflection problem, one might electrically connect repeaters 86 and 88 so that they cannot both be operative simultaneously. FIG. 4 illustrates this expedient, with conductors 90 being used to transfer inhibit signals. Thus when a stream of light pulses is emitted from star 82 to star 84, for example, repeater 86 inhibits the operation of repeater 88 so that the reflection from star 84 is not transferred by repeater 88 back to star 82. Due to propagation delays, however, repeater 88 would become operative before it had received the tail end of the signal reflected by star 84. In order to ensure reliable operation it would be necessary to deactivate repeater 88 for an additional period following the period in which repeater 86 was operative. This increased delay would be significant if long fiber lengths are involved. Moreover, for signals propagating through repeater 88, a different period of deactivation might be required for repeater 86, since this would depend on the signal propagation time from repeater to repeater through star 82, which might be at a different distance from the repeaters than star 84.
The requirement to inhibit data transmission in one direction at the end of each message tends to complicate the use of repeaters. If the inhibit period is lengthened to a value corresponding to a specified maximum propagation time, then it is necessary to ensure that no terminal devices begin a new message during this extended inhibit period. This of course would complicate the network protocols employed in the network control system. Moreover, it might also be necessary to set different inhibit periods for each pair of repeaters, depending upon their location between the stars.